RFC 1267

Border Gateway Protocol 3 (BGP-3)

Network Working Group K. Lougheed
Request for Comments: 1267 cisco Systems
Obsoletes RFCs: 1105, 1163 Y. Rekhter
T.J. Watson Research Center, IBM Corp.
October 1991 A Border Gateway Protocol 3 (BGP-3)
Status of this Memo
This memo, together with its companion document, "Application of the
Border Gateway Protocol in the Internet", define an inter-autonomous
system routing protocol for the Internet. This RFC specifies an IAB
standards track protocol for the Internet community, and requests
discussion and suggestions for improvements. Please refer to the
current edition of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol. Distribution of
this memo is unlimited.
1. Acknowledgements
We would like to express our thanks to Guy Almes (Rice University),
Len Bosack (cisco Systems), Jeffrey C. Honig (Cornell Theory Center)
and all members of the Interconnectivity Working Group of the
Internet Engineering Task Force, chaired by Guy Almes, for their
contributions to this document.
We like to explicitly thank Bob Braden (ISI) for the review of this
document as well as his constructive and valuable comments.
We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review earlier versions of this document. This team,
consisting of Deborah Estrin, Milo Medin, John Moy, Radia Perlman,
Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted with a
strong combination of toughness, professionalism, and courtesy.
2. Introduction
The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol. It is built on experience gained with EGP as
defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
described in RFC 1092 [2] and RFC 1093 [3].
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the full path of

Autonomous Systems (ASs) that traffic must transit to reach these
networks. This information is sufficient to construct a graph of AS
connectivity from which routing loops may be pruned and some policy
decisions at the AS level may be enforced.
To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that an AS advertize to its
neighbor ASs only those routes that it itself uses. This rule
reflects the "hop-by-hop" routing paradigm generally used throughout
the current Internet. Note that some policies cannot be supported by
the "hop-by-hop" routing paradigm and thus require techniques such as
source routing to enforce. For example, BGP does not enable one AS
to send traffic to a neighbor AS intending that that traffic take a
different route from that taken by traffic originating in the
neighbor AS. On the other hand, BGP can support any policy
conforming to the "hop-by-hop" routing paradigm. Since the current
Internet uses only the "hop-by-hop" routing paradigm and since BGP
can support any policy that conforms to that paradigm, BGP is highly
applicable as an inter-AS routing protocol for the current Internet.
A more complete discussion of what policies can and cannot be
enforced with BGP is outside the scope of this document (but refer to
the companion document discussing BGP usage [5]).
BGP runs over a reliable transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgement, and sequencing. Any authentication scheme used by
the transport protocol may be used in addition to BGP's own
authentication mechanisms. The error notification mechanism used in
BGP assumes that the transport protocol supports a "graceful" close,
i.e., that all outstanding data will be delivered before the
connection is closed.
BGP uses TCP [4] as its transport protocol. TCP meets BGP's
transport requirements and is present in virtually all commercial
routers and hosts. In the following descriptions the phrase
"transport protocol connection" can be understood to refer to a TCP
connection. BGP uses TCP port 179 for establishing its connections.
This memo uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
and common metrics to route packets within the AS, and using an
exterior gateway protocol to route packets to other ASs. Since this
classic definition was developed, it has become common for a single
AS to use several interior gateway protocols and sometimes several
sets of metrics within an AS. The use of the term Autonomous System
here stresses the fact that, even when multiple IGPs and metrics are

used, the administration of an AS appears to other ASs to have a
single coherent interior routing plan and presents a consistent
picture of what networks are reachable through it. From the
standpoint of exterior routing, an AS can be viewed as monolithic:
reachability to networks directly connected to the AS must be
equivalent from all border gateways of the AS.
The planned use of BGP in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the
enforcement of routing policy rules is presented in a companion
document [5]. This document is the first of a series of documents
planned to explore various aspects of BGP application.
Please send comments to the BGP mailing list (iwg@rice.edu).
3. Summary of Operation
Two systems form a transport protocol connection between one another.
They exchange messages to open and confirm the connection parameters.
The initial data flow is the entire BGP routing table. Incremental
updates are sent as the routing tables change. BGP does not require
periodic refresh of the entire BGP routing table. Therefore, a BGP
speaker must retain the current version of the entire BGP routing
tables of all of its peers for the duration of the connection.
KeepAlive messages are sent periodically to ensure the liveness of
the connection. Notification messages are sent in response to errors
or special conditions. If a connection encounters an error
condition, a notification message is sent and the connection is
closed.
The hosts executing the Border Gateway Protocol need not be routers.
A non-routing host could exchange routing information with routers
via EGP or even an interior routing protocol. That non-routing host
could then use BGP to exchange routing information with a border
router in another Autonomous System. The implications and
applications of this architecture are for further study.
If a particular AS has multiple BGP speakers and is providing transit
service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the interior routing protocol. A
consistent view of the routes exterior to the AS can be provided by
having all BGP speakers within the AS maintain direct BGP connections
with each other. Using a common set of policies, the BGP speakers
arrive at an agreement as to which border routers will serve as
exit/entry points for particular networks outside the AS. This
information is communicated to the AS's internal routers, possibly
via the interior routing protocol. Care must be taken to ensure that

the interior routers have all been updated with transit information
before the BGP speakers announce to other ASs that transit service is
being provided.
Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links.
4. Message Formats
This section describes message formats used by BGP.
Messages are sent over a reliable transport protocol connection. A
message is processed only after it is entirely received. The maximum
message size is 4096 octets. All implementations are required to
support this maximum message size. The smallest message that may be
sent consists of a BGP header without a data portion, or 19 octets.
4.1 Message Header Format
Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The
layout of these fields is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker:
This 16-octet field contains a value that the receiver of the
message can predict. If the Type of the message is OPEN, or if
the Authentication Code used in the OPEN message of the connection
is zero, then the Marker must be all ones. Otherwise, the value
of the marker can be predicted by some a computation specified as
part of the authentication mechanism used. The Marker can be used
to detect loss of synchronization between a pair of BGP peers, and
to authenticate incoming BGP messages.

Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header, in octets. Thus, e.g., it allows
one to locate in the transport-level stream the (Marker field of
the) next message. The value of the Length field must always be
at least 19 and no greater than 4096, and may be further
constrained, depending on the message type. No "padding" of extra
data after the message is allowed, so the Length field must have
the smallest value required given the rest of the message.
Type:
This 1-octet unsigned integer indicates the type code of the
message. The following type codes are defined:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
4.2 OPEN Message Format
After a transport protocol connection is established, the first
message sent by each side is an OPEN message. If the OPEN message is
acceptable, a KEEPALIVE message confirming the OPEN is sent back.
Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
messages may be exchanged.
In addition to the fixed-size BGP header, the OPEN message contains
the following fields:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 3.
My Autonomous System:
This 2-octet unsigned integer indicates the Autonomous System
number of the sender.
Hold Time:
This 2-octet unsigned integer indicates the maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE and/or UPDATE and/or NOTIFICATION messages.
BGP Identifier:
This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP
Identifier to the IP address of one of its interfaces.
The value of the BGP Identifier is determined on startup
and is the same for every local interface and every BGP peer.
Authentication Code:
This 1-octet unsigned integer indicates the authentication
mechanism being used. Whenever an authentication mechanism is
specified for use within BGP, three things must be included in the
specification:

- the value of the Authentication Code which indicates use of
the mechanism,
- the form and meaning of the Authentication Data, and
- the algorithm for computing values of Marker fields.
Only one authentication mechanism is specified as part of this
memo:
- its Authentication Code is zero,
- its Authentication Data must be empty (of zero length), and
- the Marker fields of all messages must be all ones.
The semantics of non-zero Authentication Codes lies outside the
scope of this memo.
Note that a separate authentication mechanism may be used in
establishing the transport level connection.
Authentication Data:
The form and meaning of this field is a variable-length field
depend on the Authentication Code. If the value of Authentication
Code field is zero, the Authentication Data field must have zero
length. The semantics of the non-zero length Authentication Data
field is outside the scope of this memo.
Note that the length of the Authentication Data field can be
determined from the message Length field by the formula:
Message Length = 29 + Authentication Data Length
The minimum length of the OPEN message is 29 octets (including
message header).
4.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE packet can be used to construct
a graph describing the relationships of the various Autonomous
Systems. By applying rules to be discussed, routing information
loops and some other anomalies may be detected and removed from
inter-AS routing.
In addition to the fixed-size BGP header, the UPDATE message contains
the following fields (note that all fields may have arbitrary
alignment):

The third high-order bit (bit 2) of the Attribute Flags octet is
the Partial bit. It defines whether the information contained in
the optional transitive attribute is partial (if set to 1) or
complete (if set to 0). For well-known attributes and for
optional non-transitive attributes the Partial bit must be set to
0.
The fourth high-order bit (bit 3) of the Attribute Flags octet is
the Extended Length bit. It defines whether the Attribute Length
is one octet (if set to 0) or two octets (if set to 1). Extended
Length may be used only if the length of the attribute value is
greater than 255 octets.
The lower-order four bits of the Attribute Flags octet are unused.
They must be zero (and must be ignored when received).
The Attribute Type Code octet contains the Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section 5.
If the Extended Length bit of the Attribute Flags octet is set to
0, the third octet of the Path Attribute contains the length of
the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set to
1, then the third and the fourth octets of the path attribute
contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the attribute
value and are interpreted according to the Attribute Flags and the
Attribute Type Code.
The meaning and handling of Path Attributes is discussed in
Section 5.
Network:
Each 4-octet Internet network number indicates one network whose
Inter-Autonomous System routing is described by the Path
Attributes. Subnets and host addresses are specifically not
allowed. The total number of Network fields in the UPDATE message
can be determined by the formula:
Message Length = 19 + Total Path Attribute Length + 4 * #Nets
The message Length field of the message header and the Path
Attributes Length field of the UPDATE message must be such that
the formula results in a non-negative integer number of Network
fields.

The minimum length of the NOTIFICATION message is 21 octets
(including message header).
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories:
1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.
Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every
UPDATE message. Others are discretionary and may or may not be sent
in a particular UPDATE message. Which well-known attributes are
mandatory or discretionary is noted in the table below.
All well-known attributes must be passed along (after proper
updating, if necessary) to other BGP peers.
In addition to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected that all
BGP implementations support all optional attributes. The handling of
an unrecognized optional attribute is determined by the setting of
the Transitive bit in the attribute flags octet. Paths with
unrecognized transitive optional attributes should be accepted. If a
path with unrecognized transitive optional attribute is accepted and
passed along to other BGP peers, then the unrecognized transitive
optional attribute of that path must be passed along with the path to
other BGP peers with the Partial bit in the Attribute Flags octet set
to 1. If a path with recognized transitive optional attribute is
accepted and passed along to other BGP peers and the Partial bit in
the Attribute Flags octet is set to 1 by some previous AS, it is not
set back to 0 by the current AS. Unrecognized non-transitive optional
attributes must be quietly ignored and not passed along to other BGP
peers.
New transitive optional attributes may be attached to the path by the
originator or by any other AS in the path. If they are not attached
by the originator, the Partial bit in the Attribute Flags octet is
set to 1. The rules for attaching new non-transitive optional
attributes will depend on the nature of the specific attribute. The
documentation of each new non-transitive optional attribute will be
expected to include such rules. (The description of the INTER-AS
METRIC attribute gives an example.) All optional attributes (both

transitive and non-transitive) may be updated (if appropriate) by ASs
in the path.
The sender of an UPDATE message should order path attributes within
the UPDATE message in ascending order of attribute type. The
receiver of an UPDATE message must be prepared to handle path
attributes within the UPDATE message that are out of order.
The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message.
Following table specifies attribute type code, attribute length, and
attribute category for path attributes defined in this document:
Attribute Name Type Code Length Attribute category
ORIGIN 1 1 well-known, mandatory
AS_PATH 2 variable well-known, mandatory
NEXT_HOP 3 4 well-known, mandatory
UNREACHABLE 4 0 well-known, discretionary
INTER-AS METRIC 5 2 optional, non-transitive
ORIGIN:
The ORIGIN path attribute defines the origin of the path
information. The data octet can assume the following values:
Value Meaning
0 IGP - network(s) are interior to the originating AS
1 EGP - network(s) learned via EGP
2 INCOMPLETE - network(s) learned by some other means
AS_PATH:
The AS_PATH attribute enumerates the ASs that must be traversed to
reach the networks listed in the UPDATE message. Since an AS
identifier is 2 octets, the length of an AS_PATH attribute is
twice the number of ASs in the path. Rules for constructing an
AS_PATH attribute are discussed in Section 9.
If a previously advertised route has become unreachable, then
the AS_PATH path attribute of the unreachable route may be
truncated when passed in the UPDATE message. Truncation is
achieved by constructing the AS_PATH path attribute that consists
of only the autonomous system of the sender of the UPDATE message.
To make the truncated AS_PATH semantically correct, the sender
also sends the ORIGIN path attribute with the value INCOMPLETE.
Note that truncation may be done only over external BGP links.

NEXT_HOP:
The NEXT_HOP path attribute defines the IP address of the border
router that should be used as the next hop to the networks listed
in the UPDATE message. If this border router belongs to the same
AS as the BGP peer that advertises it, it is called an internal
border router. If this border router belongs to a different AS
than the one that the BGP peer that advertises it, it is called an
external border router. A BGP speaker can advertise any internal
border router as the next hop provided that the interface
associated with the IP address of this border router (as
specified in the NEXT_HOP path attribute) shares a common subnet
with both the local and remote BGP speakers. A BGP speaker can
advertise any external border router as the next hop, provided
that the IP address of this border router was learned from one
of the BGP speaker's peers, and the interface associated with
the IP address of this border router (as specified in the
NEXT_HOP path attribute) shares a common subnet with the local
and remote BGP speakers. A BGP speaker needs to be able to
support disabling advertisement of external border routers.
The NEXT_HOP path attribute has meaning only on external BGP
links. However, presence of the NEXT_HOP path attribute in the
UPDATE message received via an internal BGP link does not
constitute an error.
UNREACHABLE:
The UNREACHABLE attribute is used to notify a BGP peer that some
of the previously advertised routes have become unreachable.
INTER-AS METRIC:
The INTER-AS METRIC attribute may be used on external (inter-AS)
links to discriminate between multiple exit or entry points to the
same neighboring AS. The value of the INTER-AS METRIC attribute
is a 2-octet unsigned number which is called a metric. All other
factors being equal, the exit or entry point with lower metric
should be preferred. If received over external links, the INTER-
AS METRIC attribute may be propagated over internal links to other
BGP speaker within the same AS. The INTER-AS METRIC attribute is
never propagated to other BGP speakers in neighboring AS's.
If a previously advertised route has become unreachable, then
the INTER-AS METRIC path attribute may be omitted from the UPDATE
message.

6. BGP Error Handling.
This section describes actions to be taken when errors are detected
while processing BGP messages.
When any of the conditions described here are detected, a
NOTIFICATION message with the indicated Error Code, Error Subcode,
and Data fields is sent, and the BGP connection is closed. If no
Error Subcode is specified, then a zero must be used.
The phrase "the BGP connection is closed" means that the transport
protocol connection has been closed and that all resources for that
BGP connection have been deallocated. Routing table entries
associated with the remote peer are marked as invalid. The fact that
the routes have become invalid is passed to other BGP peers before
the routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION
message that is sent to indicate an error is empty.
6.1 Message Header error handling.
All errors detected while processing the Message Header are indicated
by sending the NOTIFICATION message with Error Code Message Header
Error. The Error Subcode elaborates on the specific nature of the
error.
The expected value of the Marker field of the message header is all
ones if the message type is OPEN. The expected value of the Marker
field for all other types of BGP messages determined based on the
Authentication Code in the BGP OPEN message and the actual
authentication mechanism (if the Authentication Code in the BGP OPEN
message is non-zero). If the Marker field of the message header is
not the expected one, then a synchronization error has occurred and
the Error Subcode is set to Connection Not Synchronized.
If the Length field of the message header is less than 19 or greater
than 4096, or if the Length field of an OPEN message is less than
the minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE message,
or if the Length field of a KEEPALIVE message is not equal to 19, or
if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message, then the Error Subcode is
set to Bad Message Length. The Data field contains the erroneous
Length field.
If the Type field of the message header is not recognized, then the
Error Subcode is set to Bad Message Type. The Data field contains

the erroneous Type field.
6.2 OPEN message error handling.
All errors detected while processing the OPEN message are indicated
by sending the NOTIFICATION message with Error Code OPEN Message
Error. The Error Subcode elaborates on the specific nature of the
error.
If the version number contained in the Version field of the received
OPEN message is not supported, then the Error Subcode is set to
Unsupported Version Number. The Data field is a 2-octet unsigned
integer, which indicates the largest locally supported version number
less than the version the remote BGP peer bid (as indicated in the
received OPEN message).
If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode is set to Bad Peer AS. The determination of
acceptable Autonomous System numbers is outside the scope of this
protocol.
If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode is set to Bad BGP Identifier.
Syntactic correctness means that the BGP Identifier field represents
a valid IP host address.
If the Authentication Code of the OPEN message is not recognized,
then the Error Subcode is set to Unsupported Authentication Code. If
the Authentication Code is zero, then the Authentication Data must be
of zero length. Otherwise, the Error Subcode is set to
Authentication Failure.
If the Authentication Code is non-zero, then the corresponding
authentication procedure is invoked. If the authentication procedure
(based on Authentication Code and Authentication Data) fails, then
the Error Subcode is set to Authentication Failure.
6.3 UPDATE message error handling.
All errors detected while processing the UPDATE message are indicated
by sending the NOTIFICATION message with Error Code UPDATE Message
Error. The error subcode elaborates on the specific nature of the
error.
Error checking of an UPDATE message begins by examining the path
attributes. If the Total Attribute Length is too large (i.e., if
Total Attribute Length + 21 exceeds the message Length), or if the
(non-negative integer) Number of Network fields cannot be computed as

in Section 4.3, then the Error Subcode is set to Malformed Attribute
List.
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode is set to Attribute
Flags Error. The Data field contains the erroneous attribute (type,
length and value).
If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the
Error Subcode is set to Attribute Length Error. The Data field
contains the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then
the Error Subcode is set to Missing Well-known Attribute. The Data
field contains the Attribute Type Code of the missing well-known
attribute.
If any of the mandatory well-known attributes are not recognized,
then the Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the unrecognized attribute (type, length and
value).
If the ORIGIN attribute has an undefined value, then the Error
Subcode is set to Invalid Origin Attribute. The Data field contains
the unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically or semantically
incorrect, then the Error Subcode is set to Invalid NEXT_HOP
Attribute.
The Data field contains the incorrect attribute (type, length and
value). Syntactic correctness means that the NEXT_HOP attribute
represents a valid IP host address. Semantic correctness applies
only to the external BGP links. It means that the interface
associated with the IP address, as specified in the NEXT_HOP
attribute, shares a common subnet with the receiving BGP speaker.
The AS route specified by the AS_PATH attribute is checked for AS
loops. AS loop detection is done by scanning the full AS route (as
specified in the AS_PATH attribute) and checking that each AS occurs
at most once. If a loop is detected, then the Error Subcode is set
to AS Routing Loop. The Data field contains the incorrect attribute
(type, length and value).
If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is
discarded, and the Error Subcode is set to Optional Attribute Error.

The Data field contains the attribute (type, length and value).
If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List.
Each Network field in the UPDATE message is checked for syntactic
validity. If the Network field is syntactically incorrect, or
contains a subnet or a host address, then the Error Subcode is set to
Invalid Network Field.
6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an
unrecognized Error Code or Error Subcode, should be noticed, logged
locally, and brought to the attention of the administration of the
peer. The means to do this, however, lies outside the scope of this
document.
6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code must be sent and the BGP connection
closed.
6.6 Finite State Machine error handling.
Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error.
6.7 Cease.
In absence of any fatal errors (that are indicated in this section),
a BGP peer may choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message must not be used when a fatal error
indicated by this section does exist.
6.8 Connection collision detection.
If a pair of BGP speakers try simultaneously to establish a TCP
connection to each other, then two parallel connections between this
pair of speakers might well be formed. We refer to this situation as
connection collision. Clearly, one of these connections must be

closed.
Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a
collision does occur. The convention is to compare the BGP
Identifiers of the peers involved in the collision and to retain only
the connection initiated by the BGP speaker with the higher-valued
BGP Identifier.
Upon receipt of an OPEN message, the local system must examine all of
its connections that are in the OpenSent state. If among them there
is a connection to a remote BGP speaker whose BGP Identifier equals
the one in the OPEN message, then the local system performs the
following collision resolution procedure:
1. The BGP Identifier of the local system is compared to the
BGP Identifier of the remote system (as specified in the
OPEN message).
2. If the value of the local BGP Identifier is less than the
remote one, the local system closes BGP connection that
already exists (the one that is already in the OpenSent
state), and accepts BGP connection initiated by the remote
system.
3. Otherwise, the local system closes newly created BGP
connection (the one associated with the newly received OPEN
message), and continues to use the existing one (the one
that is already in the OpenSent state).
Comparing BGP Identifiers is done by treating them as
(4-octet long) unsigned integers.
A connection collision with existing BGP connections that
are either in OpenConfirm or Established states causes
unconditional closing of the newly created connection. Note
that a connection collision cannot be detected with
connections that are in Idle, or Connect, or Active states.
Closing the BGP connection (that results from the collision
resolution procedure) is accomplished by sending the
NOTIFICATION message with the Error Code Cease.
7. BGP Version Negotiation.
BGP speakers may negotiate the version of the protocol by making
multiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error

Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common
versions, then this will allow them to rapidly determine the highest
common version. In order to support BGP version negotiation, future
versions of BGP must retain the format of the OPEN and NOTIFICATION
messages.
8. BGP Finite State machine.
This section specifies BGP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of BGP
operations by state as determined by this FSM. A condensed version
of the BGP FSM is found in Appendix 1.
Initially BGP is in the Idle state.
Idle state:
In this state BGP refuses all incoming BGP connections. No
resources are allocated to the BGP neighbor. In response to
the Start event (initiated by either system or operator) the
local system initializes all BGP resources, starts the
ConnectRetry timer, initiates a transport connection to other
BGP peer, while listening for connection that may be initiated
by the remote BGP peer, and changes its state to Connect.
The exact value of the ConnectRetry timer is a local matter,
but should be sufficiently large to allow TCP initialization.
Any other event received in the Idle state is ignored.
Connect state:
In this state BGP is waiting for the transport protocol
connection to be completed.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, and changes its state to OpenSent.
If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by
the remote BGP peer, and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local

system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
stays in the Connect state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
Active state:
In this state BGP is trying to acquire a BGP neighbor by
initiating a transport protocol connection.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its hold timer to a large
value, and changes its state to OpenSent.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be be initiated by the remote BGP peer, and
changes its state to Connect.
If the local system detects that a remote peer is trying to
establish BGP connection to it, and the IP address of the
remote peer is not an expected one, the local system restarts
the ConnectRetry timer, rejects the attempted connection,
continues to listen for a connection that may be initiated by
the remote BGP peer, and stays in the Active state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
OpenSent state:
In this state BGP waits for an OPEN message from its peer.
When an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking or OPEN
message checking detects an error (see Section 6.2), or
a connection collision (see Section 6.8) the local
system sends a NOTIFICATION message and changes its state to

Idle.
If there are no errors in the OPEN message, BGP sends a
KEEPALIVE message and sets a KeepAlive timer. The hold timer,
which was originally set to an arbitrary large value (see
above), is replaced with the value indicated in the OPEN
message. If the value of the Autonomous System field is the
same as our own, then the connection is "internal" connection;
otherwise, it is "external". (This will effect UPDATE
processing as described below.) Finally, the state is changed
to OpenConfirm.
If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and
goes into the Active state.
If the hold time expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its
state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenSent state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from OpenSent to Idle, it closes
the BGP (and transport-level) connection and releases all
resources associated with that connection.
OpenConfirm state:
In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.
If the local system receives a KEEPALIVE message, it changes
its state to Established.
If the hold timer expires before a KEEPALIVE message is
received, the local system sends NOTIFICATION message with
error code Hold Timer expired and changes its state to Idle.

If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenConfirm state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from OpenConfirm to Idle, it
closes the BGP (and transport-level) connection and releases
all resources associated with that connection.
Established state:
In the Established state BGP can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer.
If the local system receives an UPDATE or KEEPALIVE message, it
restarts its Holdtime timer.
If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the local system receives an UPDATE message and the UPDATE
message error handling procedure (see Section 6.3) detects an
error, the local system sends a NOTIFICATION message and
changes its state to Idle.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to
Idle.
If the Holdtime timer expires, the local system sends a
NOTIFICATION message with Error Code Hold Timer Expired and
changes its state to Idle.
If the KeepAlive timer expires, the local system sends a

KEEPALIVE message and restarts its KeepAlive timer.
Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer.
In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the Established state.
In response to any other event, the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from Established to Idle, it
closes the BGP (and transport-level) connection, releases all
resources associated with that connection, and deletes all
routes derived from that connection.
9. UPDATE Message Handling
An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is
quietly ignored. If an optional transitive attribute is
unrecognized, the Partial bit (the third high-order bit) in the
attribute flags octet is set to 1, and the attribute is retained for
propagation to other BGP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible
propagation to other BGP speakers.
If the network and the path attributes associated with a route to
that network are correct, then the route is compared with other
routes to the same network.
When a BGP speaker receives a new route from a peer over external BGP
link, it shall advertise that route to other BGP speakers in its
autonomous system by means of an UPDATE message if either of the
following conditions occur:
a) the newly received route is considered to be better
than the other routes to the same network (as listed

in the UPDATE message) that have been received over
external BGP links, or
b) there are no other acceptable routes to the network
(as listed in the UPDATE message) that have been
received over external BGP links.
When a BGP speaker receives an unreachable route from a BGP peer over
external BGP link, it shall advertise that route to all other BGP
speakers in its autonomous system, indicating that it has become
unreachable, if the following condition occur:
a) a corresponding acceptable route to the same destination
was considered to be the best one among all routes to that
destination that have been received over external BGP links
(that is the local system has been advertising the
route to all other BGP speakers in its autonomous system
before it received the UPDATE message that reported it
as unreachable).
Whenever a BGP speaker selects a new route (among all the routes
received from external and internal BGP peers), or determines that
the reachable destinations within its own autonomous system have
changed, it shall generate an UPDATE message and forward it to each
of its external peers (peers connected via external BGP links).
If a route in the UPDATE was received over an internal link, it is
not propagated over any other internal link. This restriction is due
to the fact that all BGP speakers within a single AS form a
completely connected graph (see above).
If the UPDATE message is propagated over an external link, then the
local AS number is prepended to the AS_PATH attribute, and the
NEXT_HOP attribute is updated with an IP address of the router that
should be used as a next hop to the network. If the UPDATE message
is propagated over an internal link, then the AS_PATH attribute and
the NEXT_HOP attribute are passed unmodified.
Generally speaking, the rules for comparing routes among several
alternatives are outside the scope of this document. There are two
exceptions:
- If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than
any other route. If such a route were ever used, a routing loop
would result.
- In order to achieve successful distributed operation, only routes

with a likelihood of stability can be chosen. Thus, an AS must
avoid using unstable routes, and it must not make rapid
spontaneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require
experience, but the principle is clear.
10. Detection of Inter-AS Policy Contradictions
Since BGP requires no central authority for coordinating routing
policies among ASs, and since routing policies are not exchanged via
the protocol itself, it is possible for a group of ASs to have a set
of routing policies that cannot simultaneously be satisfied. This
may cause an indefinite oscillation of the routes in this group of
ASs.
To help detect such a situation, all BGP speakers must observe the
following rule. If a route to a destination that is currently used
by the local system is determined to be unreachable (e.g., as a
result of receiving an UPDATE message for this route with the
UNREACHABLE attribute), then, before switching to another route, this
local system must advertize this route as unreachable to all the BGP
neighbors to which it previously advertized this route.
This rule will allow other ASs to distinguish between two different
situations:
- The local system has chosen to use a new route because the old
route become unreachable.
- The local system has chosen to use a new route because it
preferred it over the old route. The old route is still
viable.
In the former case, an UPDATE message with the UNREACHABLE attribute
will be received for the old route. In the latter case it will not.
In some cases, this may allow a BGP speaker to detect the fact that
its policies, taken together with the policies of some other AS,
cannot simultaneously be satisfied. For example, consider the
following situation involving AS A and its neighbor AS B. B
advertises a route with a path of the form <B,...>, where A is not
present in the path. A then decides to use this path, and advertises
<A,B,...> to all its neighbors. B later advertises <B,...,A,...>
back to A, without ever declaring its previous path <B,...> to be
unreachable. Evidently, A prefers routes via B and B prefers routes
via A. The combined policies of A and B, taken together, cannot be
satisfied. Such an event should be noticed, logged locally, and
brought to the attention of AS A's administration. The means to do

this, however, lies outside the scope of this document. Also outside
the document is a more complete procedure for detecting such
contradictions of policy.
While the above rules provide a mechanism to detect a set of routing
policies that cannot be satisfied simultaneously, the protocol itself
does not provide any mechanism for suppressing the route oscillation
that may result from these unsatisfiable policies. The reason for
doing this is that routing policies are viewed as external to the
protocol and as determined by the local AS administrator.
Appendix 1. BGP FSM State Transitions and Actions.
This Appendix discusses the transitions between states in the BGP FSM
in response to BGP events. The following is the list of these states
and events.
BGP States:
1 - Idle
2 - Connect
3 - Active
4 - OpenSent
5 - OpenConfirm
6 - Established
BGP Events:
1 - BGP Start
2 - BGP Stop
3 - BGP Transport connection open
4 - BGP Transport connection closed
5 - BGP Transport connection open failed
6 - BGP Transport fatal error
7 - ConnectRetry timer expired
8 - Holdtime timer expired
9 - KeepAlive timer expired
10 - Receive OPEN message
11 - Receive KEEPALIVE message
12 - Receive UPDATE messages
13 - Receive NOTIFICATION message
The following table describes the state transitions of the BGP FSM
and the actions triggered by these transitions.

Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD.
The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol.
The changes in the message format from RFC1105 are as follows:
1. The Hold Time field has been removed from the BGP header and
added to the OPEN message.
2. The version field has been removed from the BGP header and
added to the OPEN message.
3. The Link Type field has been removed from the OPEN message.
4. The OPEN CONFIRM message has been eliminated and replaced
with implicit confirmation provided by the KEEPALIVE message.
5. The format of the UPDATE message has been changed
significantly. New fields were added to the UPDATE message
to support multiple path attributes.
6. The Marker field has been expanded and its role broadened to
support authentication.
Note that quite often BGP, as specified in RFC 1105, is referred to
as BGP-1, BGP, as specified in RFC 1163, is referred to as BGP-2, and
BGP, as specified in this document is referred to as BGP-3.
Appendix 4. TCP options that may be used with BGP
If a local system TCP user interface supports TCP PUSH function, then
each BGP message should be transmitted with PUSH flag set. Setting
PUSH flag forces BGP messages to be transmitted promptly to the
receiver.
If a local system TCP user interface supports setting precedence for
TCP connection, then the BGP transport connection should be opened
with precedence set to Internetwork Control (110) value (see also
[6]).

Appendix 5. Implementation Recommendations
This section presents some implementation recommendations.
5.1 Multiple Networks Per Message
The BGP protocol allows for multiple networks with the same AS path
and next-hop gateway to be specified in one message. Making use of
this capability is highly recommended. With one network per message
there is a substantial increase in overhead in the receiver. Not only
does the system overhead increase due to the reception of multiple
messages, but the overhead of scanning the routing table for flash
updates to BGP peers and other routing protocols (and sending the
associated messages) is incurred multiple times as well. One method
of building messages containing many networks per AS path and gateway
from a routing table that is not organized per AS path is to build
many messages as the routing table is scanned. As each network is
processed, a message for the associated AS path and gateway is
allocated, if it does not exist, and the new network is added to it.
If such a message exists, the new network is just appended to it. If
the message lacks the space to hold the new network, it is
transmitted, a new message is allocated, and the new network is
inserted into the new message. When the entire routing table has been
scanned, all allocated messages are sent and their resources
released. Maximum compression is achieved when all networks share a
gateway and common path attributes, making it possible to send many
networks in one 4096-byte message.
When peering with a BGP implementation that does not compress
multiple networks into one message, it may be necessary to take steps
to reduce the overhead from the flood of data received when a peer is
acquired or a significant network topology change occurs. One method
of doing this is to limit the rate of flash updates. This will
eliminate the redundant scanning of the routing table to provide
flash updates for BGP peers and other routing protocols. A
disadvantage of this approach is that it increases the propagation
latency of routing information. By choosing a minimum flash update
interval that is not much greater than the time it takes to process
the multiple messages this latency should be minimized. A better
method would be to read all received messages before sending updates.
5.2 Processing Messages on a Stream Protocol
BGP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as
messages, especially on systems such as BSD Unix where it is not
possible to determine how much data has been received but not yet

processed.
One method that can be used in this situation is to first try to read
just the message header. For the KEEPALIVE message type, this is a
complete message; for other message types, the header should first be
verified, in particular the total length. If all checks are
successful, the specified length, minus the size of the message
header is the amount of data left to read. An implementation that
would "hang" the routing information process while trying to read
from a peer could set up a message buffer (4096 bytes) per peer and
fill it with data as available until a complete message has been
received.
5.3 Processing Update Messages
In BGP, all UPDATE messages are incremental. Once a particular
network is listed in an Update message as being reachable through an
AS path and gateway, that piece of information is expected to be
retained indefinitely.
In order for a route to a network to be removed, it must be
explicitly listed in an Update message as being unreachable or with
new routing information to replace the old. Note that a BGP peer will
only advertise one route to a given network, so any announcement of
that network by a particular peer replaces any previous information
about that network received from the same peer.
One useful optimization is that unreachable networks need not be
advertised with their original attributes. Instead, all unreachable
networks could be sent in a single message, perhaps with an AS path
consisting of the local AS only and with an origin set to INCOMPLETE.
This approach has the obvious advantage of low overhead; if all
routes are stable, only KEEPALIVE messages will be sent. There is no
periodic flood of route information.
However, this means that a consistent view of routing information
between BGP peers is only possible over the course of a single
transport connection, since there is no mechanism for a complete
update. This requirement is accommodated by specifying that BGP peers
must transition to the Idle state upon the failure of a transport
connection.
5.4 BGP Timers
BGP employs three timers: ConnectRetry, Holdtime, and KeepAlive.
Suggested value for the ConnectRetry timer is 120 seconds.
Suggested value for the Holdtime timer is 90 seconds.

Suggested value for the KeepAlive timer is 30 seconds.
An implementation of BGP shall allow any of these timers to be
configurable.
5.5 Frequency of Route Selection
An implementation of BGP shall allow a border router to set up the
minimum amount of time that must elapse between selection and
subsequent advertisement of better routes received by a given BGP
speaker from BGP speakers located in adjacent ASs.
Since fast convergence is needed within an AS, deferring selection
does not apply to selection of better routes chosen as a result of
UPDATEs from BGP speakers located in the advertising speaker's own
AS. To avoid long-lived black holes, it does not apply to
advertisement of previously selected routes which have become
unreachable. In both of these situations, the local BGP speaker must
select and advertise such routes immediately.
If a BGP speaker received better routes from BGP speakers in adjacent
ASs, but have not yet advertised them because the time has not yet
elapsed, the reception of any routes from other BGP speakers in its
own AS shall trigger a new route selection process that will be based
on both updates from BGP speakers in the same AS and in adjacent ASs.
References
[1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC904, BBN, April 1984.
[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
Backbone", RFC 1092, T.J. Watson Research Center, February 1989.
[3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
MERIT/NSFNET Project, February 1989.
[4] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, DARPA, September 1981.
[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC 1268, T.J. Watson Research Center,
IBM Corp., ANS, October 1991.
[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", RFC 791, DARPA, September 1981.